Development of a Water Purification Method by Injecting Surface Water to Hypolimnion in Dams and Reservoirs

在富营养化的湖泊和水库中观察到的溶解氧(DO)枯竭，损害了自然底栖生物，并诱导了沉积物中的营养物质释放。针对这一问题，提出了一种向DO经常耗尽的底层注入大量DO的地表水注入方法。在这个系统中，注入地下离子的地表水通过热交换器冷却到地下离子的水温，并在沉积物上方形成一层薄薄的氧化层。因此，通过运行这个系统，可以抑制沉积物中营养物质的释放。利用该系统向地下注入地表水对水质的影响已经在小范围内得到了验证。为此，本研究在面积为6.0平方公里的岩手县塔斯湖，通过野外试验，研究了地下水连续注入情况下的水质恢复程度。2001年的实验结果表明，在距离实验设备至少20m的范围内，亚极化离子中的DO浓度得到了改善。在此之后，对一些装置进行了改进，从而在距离实验装置至少40m的范围内，于2002年观察到了对次生离子中DO浓度提高的扩大效应。在DO浓度提高期间，PO4~+-P浓度持续下降，在系统安装点最低。随着NH4+-N浓度的降低，硝化作用也表现出来。这些结果表明，向底层注入地表水可以使底层水质得到净化，即使在塔斯湖这样的大范围水域，适当持续地注入地表水也可以扩大其净水能力。

Dissolved oxygen (DO) depletion, observed in eutrophic lakes and reservoirs, damages natural benthic organisms and induces nutrient release from the sediment. For this problem, a method which injects surface water, including a large amount of DO into the bottom layer in which DO is often depleted, is proposed. In this system, surface water injected into hypolimnion is cooled down to a water temperature of hypolimnion by a heat exchanger, and forms a thin oxic layer immediately above the sediment. As a result, nutrients release from the sediment could be suppressed by operating this system. Effects of surface-water injection into hypolimnion on water quality by this system had been already proved in small-scale area. In this study, therefore, we investigated the degree of water-quality restoration in case of continuous surface-water injection in hypolimnion by field experiments in Lake Tase in Iwate Prefecture, which has 6.0km^2 surface area. Experimental results in 2001 showed that DO concentration in hypolimnion was improved within at least 20m from the experimental equipment. After that, some devices were improved, therefore, expanded effect on the enhancement of DO concentration in hypolimnion was observed within at least 40m from the experimental equipment in 2002. In the period when DO concentration had been improved, PO_4^<3->-P concentration in hypolimnion kept decreasing and it was lowest at the system-installed point. The nitrification was also showed, accompanied by decreasing of NH_4^+-N concentration. These results suggested that injection of surface water into the hypolimnion made water quality in the bottom layer purified, and that appropriate and continuous injection of surface water could expand its water-purification ability, even in a large-scale water area like Lake Tase.